1. Field of the Invention
The present invention relates to a detector for detecting information of the displacement of a moving object or fluid (hereinafter referred to as moving object), or a velocimeter for measuring the velocity of a moving object without contacting the object, such as a laser Doppler velocimeter for the measurement by detecting the frequency shift of laser light.
2. Related Background Art
Traditionally, the laser Doppler velocimeter has been in use as an apparatus for performing a noncontact high-precision measurement of the traveling velocity of a moving object. The laser Doppler velocimeter is an apparatus for measuring the traveling velocity of the moving object by the utilization of the effect (Doppler effect) that the frequency of scattering light by the moving object is shifted in proportion to its traveling velocity when light beam such as laser light is radiated onto the moving object.
An example of the conventional laser Doppler velocimeter is schematically shown in FIG. 1 to illustrate its structure.
In FIG. 1, a reference numeral 1 designates a laser light source; 2, a collimator lens; 3, parallel beam; 4, a beam splitter; 6a and 6b, reflection mirrors; 7, a moving object in the direction indicated by an arrow at a traveling velocity V; 8, a condenser lens; and 9, an photodetector.
The laser light emitted from the laser light source 1 is produced into the parallel beam 3 by the function of the collimator lens 2, and splitted into two beams 5a and 5b by the beam splitter 4. The splitted beams 5a, 5b are reflected by the reflection mirrors 6a and 6b, respectively, and are irradiated at an incident angle .theta. by the two-beam irradiation onto one and a same position on the moving object 7 at the traveling velocity V. The scattering light from the moving object at that time is detected by the photodetector 9 through the condenser lens 8. The frequency of the scattering light by the two beams is proportional to the traveling velocity V and affected by the Doppler shift of +.DELTA.f and -.DELTA.f, respectively. Now, given the wavelength of the laser light as .lambda., the .DELTA.f can be expressed in an equation (1) given below. EQU .DELTA.f=V.multidot.sin(.theta.)/.lambda. (1)
The scattering lights affected by the Doppler shifts, +.DELTA.f and -.DELTA.f, interfere with each other to result in the variation of light intensity on the light receiving plane of the photodetector 9. Its frequency F is obtainable by an equation (2) given below. EQU F=2.multidot..DELTA.f=2.multidot.V.multidot.sin(.theta.)/.lambda.(2)
With the measurement of the frequency F (hereinafter referred to as Doppler frequency) of the photodetector 9 by the equation (2), the traveling velocity V of the moving object 7 can be obtained.
In such a laser Doppler velocimeter as in the above example of the conventional art, the Doppler frequency F is inversely proportional to the laser wavelength .lambda. as understandable from the equation (2). Therefore, it is necessary to use a laser light source capable of emitting a light having stable wavelength for the laser Doppler velocimeter. As a laser light source capable of continuous transmission with a stable wavelength, a gas laser such as He-Ne is often employed, but the size of its laser oscillator is large and a high voltage power source is needed. Accordingly, the apparatus becomes large and expensive. On the other hand, the laser diode (or semiconductor laser) employed for a compact disc, video disc, optical fiber communications or the like is very small and easy to drive. However, a stable output is hardly obtainable due to its temperature dependence.
FIG. 2 (quoted from Optical Semiconductor Devices compiled in Mitsubishi Semiconductor Data Book 1987) illustrates an example of the typical temperature dependence of a laser diode. The portion where the wavelength changes continuously is mainly caused by the temperature changes of the refractive index of the active layer of the laser diode, which is 0.05-0.06 nm/.degree.C. The portion where the wavelength changes discontinuously is called longitudinal mode hopping, which is 0.2-0.3 nm/.degree.C.
In order to stabilize the wavelength, a controlling method is usually adopted to maintain the laser diode at a constant temperature. Such method requires the installation of the temperature control members such as heater, radiator, and temperature sensor with small thermal resistance to control the temperature precisely. This makes the laser Doppler velocimeter comparatively large in its size and high in its cost, in addition, it is still impossible to eliminate completely the unstableness resulting from the longitudinal mode hopping.
With a view to solving the above-mentioned problem, a method for detecting the scattering lights from a moving object or fluid by the use of a photodetector (hereinafter referred to as G-LDV) has been proposed, for example, in U.S. Ser. No. 501,499 as a laser Doppler velocimeter, in which the laser light as the light source is incident on a diffraction grating, and the two of the obtainable diffracted lights, which are +n order and -n order (n is 1, 2, and, . . . ) except zero order, are irradiated onto the moving object at an intersecting angle which is same angle formed by the two-beam irradiation.
FIG. 3 illustrates an example of diffraction wherein a laser light I is incident on a transmission type diffraction grating 10 having a grating pitch d, perpendicularly to the direction t in which the grating is arrayed, and the diffraction angle .theta..sub.0 is expressed by an equation given below. EQU sin .theta..sub.0 =m.lambda./d
where m is the diffraction order (0, 1, 2, and . . . ) and .lambda. is the wavelength of light.
Here, the light of .+-.n order other than zero order is expressed by an equation given below. EQU sin .theta..sub.0 =.+-.n.lambda./d (3)
where n is 1, 2, and . . .
FIG. 4 is a view illustrating the two-beam irradiation of said .+-.n order light onto the moving object 7 by the use of the mirrors 6a and 6b in such a manner that its incident angle becomes .theta..sub.0. The Doppler frequency F of the photodetector 9 is expressed by an equation given below, obtained from the equations (2) and (3): ##EQU1## The Doppler frequency F does not depend on the wavelength of the laser light I, and is inversely proportional to the grating pitch d of the diffraction grating 10 and is proportional to the traveling velocity V of the moving object 7. Because the grating pitch d is sufficiently stable, the frequency proportional to only the traveling velocity V of the moving object 7 is obtainable as the Doppler frequency F. In this respect, when a reflection type diffraction grating is used as the grating 10, same result will be obtained.